1. Field of the Invention
The present invention relates to a diaphragm device for projector.
2. Description of the Related Art
In various projectors including a liquid crystal projector, there is a projector that adjusts an amount of light illuminated from a light source to increase contrast by changing the amount of light illuminated according to a screen size or environmental brightness or excluding light from an outer side away from a light axis in an illumination optical system. In this case, the projector often adjusts an amount of light illuminated by not changing an amount of luminescence of the light source but instead arranging a diaphragm device in a certain position in front of a resolution optical system from the light source.
Patent document 1 (Japanese Published Unexamined Patent Application No. 2004-302108) discloses a diaphragm device for projector that moves two diaphragm plates, each of which has linear open regulation edges, in directions opposed to each other by a motor. More specifically, the diaphragm plates each of which is formed in L-letter shape are arranged between two ground plates. Two operation members each of which has arm portions at both sides thereof are respectively mounted the ground plates. Each of the operation members is rotated around an axis by the motor, which reciprocates via pins the diaphragm plates along a diagonal line of an aperture portion surrounded by the linear open regulation edges of the diaphragm plates. By this reciprocation, the linear open regulation edges of the diaphragm plates change a size of aperture for path of light illuminated.
The liquid crystal projector uses a lens array type illumination optical system composed of a first lens array and a second lens array to increase illumination efficiency and improve illuminance uniformity on a screen. A diaphragm device, which is arranged in a projector using the illumination optical system, is often configured to change an area of aperture while the aperture keeps a rectangular shape. However, when the diaphragm plates stop in a state where an edge of the aperture strides across respective lens segments of each lens array, the edge of the aperture blocks a part of light illuminated at a position where it strides across the respective lens segments. This affects an illuminance distribution on a screen. Therefore, it is desirable that the diaphragm plates stop in a state where the edge of the aperture stops along a border of each lens segment without striding across the respective lens segments.
In a conventional diaphragm device for projector such as the diaphragm device disclosed in the patent document 1, a rotation angle of a motor is substantially proportional to a movement distance of an edge of aperture. This needs to control the rotation angle of the motor at a certain angle to stop the edge of aperture along a border of each lens segment of each lens array. In addition, since a rotation axis of the motor spins around in a state where an electric source is not turned on in the nature of motor, there is a possible that the edge of aperture brings unexpected displacement due to some factors.